Conventionally, the number of vehicles with headlights changed from halogen lamps to HID (High Intensity Discharged) lamps was increased in order to improve visibility (improve brightness). Mass production of vehicles equipped with LED (light-emitting device) headlights has been started along with improvement in LED luminous efficiency in recent years. For example, JP Pub. No. 2011-050126 (hereinafter referred to as “Document 1”) discloses a lighting device configured to power LEDs as headlight loads.
The lighting device described in Document 1 includes a DC/DC converter. The DC/DC converter includes an input terminal electrically connected to a DC power supply such as an in-vehicle battery and an output terminal electrically connected to the LEDs as loads, namely light sources.
The DC/DC converter includes an input capacitor electrically connected in parallel with an input connector of the lighting device. The input capacitor is electrically connected to a series circuit of a primary winding of a transformer and a switch device formed of an MOSFET. A secondary winding of the transformer is electrically connected to an output capacitor through a diode. When the switch device is turned on, a primary-side current (a drain current of the switch device) linearly increases and electromagnetic energy is stored in the transformer. When the switch device is then turned off, the diode is conducted by counter-electromotive force of the transformer and the energy stored in the transformer is discharged into the output capacitor.
The lighting device is provided with a primary-side current detecting circuit. When the switch device is turned on, a voltage proportional to the primary-side current occurs at a drain terminal of the switch device. The primary-side current detecting circuit is configured to detect the voltage to output it as a primary-side current detection value. The primary-side current detecting circuit monitors a drain voltage of the switch device when it is turned off, and determines discharge timing of the energy stored in the transformer by detecting timing when the drain voltage decreases. A detection result thereof is transmitted to a microcomputer as a secondary-side current discharge signal.
When receiving the secondary-side current discharge signal, the microcomputer turns the switch device on. When the primary-side current detection value reaches a primary-side current command value, the microcomputer turns the switch device off. By repeating the aforementioned operations, the microcomputer controls the switch device at a boundary current mode.
There has recently been a decreasing trend in an output voltage of a DC/DC converter when powering LEDs along with realization of LEDs with high efficiency, designed for high-current operation. The output voltage of the DC/DC converter needs to be decreased in a case where a part of LEDs constituting a load breaks down and remaining LEDs are still operated. The switch device is generally selected from switch devices each of which has on-resistance as small as possible in order to reduce loss of circuit.
In the lighting device, the primary-side current detection value is however to have a small value if a switch device having a small on-resistance is used in a case where an output of the DC/DC converter is a low output voltage. There is therefore a concern that the lighting device cannot stably control an output current thereof because the primary-side current detection value has a small variation range, so that the turn-off timing of the switch device cannot be accurately detected.